1. Technical Field
The embodiments of the disclosure generally relate to ad-hoc communications networks and, more particularly, to an ad-hoc interplanetary communications network for adaptable deep-space communications in an unstructured and self-supervised interplanetary or sub-planetary environment, such as between deep-space or lunar probes and earth.
2. Background Description
The National Air and Space Administration (NASA) is planning for future outer space exploration and, especially, for deep space interplanetary exploration in our solar system and beyond. Current plans allow for a yet-to-be-designed and developed interplanetary communications network to provide communication services between space borne entities (including deep space entities) and the earth. Primarily, the interplanetary communications network is expected to provide communication services for scientific data delivery and also provide navigation services for exploration spacecraft and orbiters in future deep space missions. The current vision for the infrastructure of this interplanetary communications network is similar to the Internet. See, e.g., Akyildiz et al., “InterPlaNetary Internet: state-of-the-art and research challenges,” Computer Networks, 43 (2003). This as yet unrealized interplanetary communications network infrastructure or, Interplanetary Internet, is enabling networking technology for future deep space scientific exploration missions such as Mars and Neptune exploration and beyond.
Generally, an interplanetary communications network is expected to include communication between nodes at various space borne entities or locations, e.g., at fixed (celestially fixed) and/or mobile communications platforms. Individual nodes may include, for example, fixed (on a planet surface) sensors, and mobile nodes, e.g., robotics as well as human operated nodes. The nodes are expected to be distributed at numerous space borne locations and deep space entities. These entities may include, for example, robotic spacecraft and Crew Exploration Vehicles (CEV's); planetary platforms, e.g., orbital, local flight and surface planet (mobile and fixed) vehicles; and, sub-planetary probes, e.g., on moons, satellites, and asteroids.
Neither terrestrial Internet-based routing nor terrestrial mobile ad hoc routing protocols satisfy space communications parameter requirements because of additional constraints and requirements for space communications, such as burst data transfers between nodes in a short transfer window. A typical Earth-based wireless network includes fixed communications backbone nodes (e.g., base stations) that define cells, for example, connected together in the network. An earth network that lacks the fixed communications backbone nodes is known as an ad-hoc network. Instead, a group of autonomous (and frequently mobile) nodes define the ad-hoc Earth network. However, since there is no fixed frame of reference in space, node locations are in constant motion with respect to one another even at rest.
Consequently, backbone network structure is expected to be fluid and continually, dynamically changing, whether as a result of planetary rotation or orbital movement. Dynamically changing node locations cause connectivity among the network nodes to vary with time. Further, connectivity may change because of other interference, such as blockage of the line-of-sight communications path by a planet or from extra-network interference, e.g., sunspot activity. This continual connectivity change makes network infrastructure time varying also and difficult to pre-define, especially as the total number of nodes gets large. Thus, the interplanetary communications network is expected to be an ad-hoc network, primarily of autonomous nodes self-managing and self-maintaining connectivity in spite of the fluidity of the network communication paths.
These autonomous nodes must assure some form of network connectivity to maintain end-to-end communications for mission success. This is especially important for exchanging large volumes of data that may be collected by various space borne network platforms. Therefore, the network nodes themselves must automatically self-configure/self-provision nodes/platforms along network paths to deliver the expected volume of data. Furthermore, this must be with minimal or no manual intervention/interference, as none may be available. Given that even when a communications window is available between two nodes, there may still be a relatively long transmission path lag time or propagation delay, even between two relatively close communicating nodes, e.g., on the moon and on the Earth. Therefore, Akyildiz et al. describe several significant challenges and issues that must be addressed and resolved before interplanetary communications network objectives may be realized.
Specifically, backbone layer routing is a serious problem area with key previously unresolved challenges. Traditional Shortest Path Algorithms (SPA) include, for example, the Bellman-Ford algorithm and Dijkstra's algorithm. The Bellman-Ford algorithm has been realized by the known Internet Border Gateway Protocol (BGP). Dijkstra's algorithm has been realized by the Internet Open Shortest Path First (OSPF) protocol for Autonomous Systems (AS). The interplanetary communications network will not have a traditional end-to-end path because of long periods (minutes, hours or even days) of no connectivity between nodes and groups of nodes. End-to-end connectivity is not guaranteed and, if it occurs, it may be only sporadic. Therefore, traditional end-to-end routing approaches are unsuitable for interplanetary communications network routing. Moreover, because of nodal motion, it may be difficult to identify an end-to-end path because performance/routing metrics (e.g., propagation and connectivity metrics) are time-dependent. Consequently, optimal or suboptimal routes are time-dependent. This time-dependence makes both the Bellman-Ford algorithm and Dijkstra's algorithm inadequate.
With current technology achieving significant distances in space, such as interplanetary space travel, currently take years to reach their objectives. Thus, distant nodes are likely to be the oldest and have the oldest equipment. Consequently, for example, because storage density increases with each new generation, storage is likely to be denser and more plentiful at nodes closer to Earth and scarcer at distant nodes. Thus, storage capacity may be in short supply and, therefore, very costly at these distant nodes as well as other intervening nodes in the network paths. As a result, long term storage requirements for storing data when a connection is unavailable can cause storage contention and overflow at those distant or intervening nodes, e.g., from data arriving simultaneously from several distant nodes. Therefore, locating and planning an optimal route requires complete knowledge and consideration of network path resources as well as key time-dependent network parameters, e.g., contact times and orbital parameters, and traffic loads and node queuing delays.
Furthermore, an interplanetary communications network is likely to be an amalgamation of sub-networks that are based on different distinct network protocols, e.g. layer-3 routing protocols. These distinct network protocols must communicate with the network through strategically located gateways. However, maintaining an even data traffic flow between network nodes that are based on different distinct network protocols requires that network gateways seamlessly convert between network protocols. Since the nodes are mobile, the gateway node positions are predictable, e.g., satellites orbiting about a distant planet.
However, connections to these mobile nodes are also normally time varying. As a result, node responsibilities change from time to time, with different nodes being designated as gateway as node connections change. So, at some point in time a node may have the best position to act as gateway and assume that responsibility. Subsequently, that node may move from that location (as other nodes also move out of position) with another node having the best location and assuming gateway responsibility. While it may be relatively simple to decide at any instant which node is at the best location to act as gateway; the continual variation in node locations further complicates gateway selection and timing and managing each gateway handover from one network node to another.
Moreover, since very likely remote planetary surface nodes, for example, must be self powering, it is likely that those remote nodes self power with solar chargeable batteries. However, even a fully charged solar battery has a limited (fixed) power capability before it must be recharged. If mid-transmission, a currently selected gateway expends all of its available power (i.e., discharges its battery), the remote network is cut off from the backbone until it selects another gateway and resumes communications. While data may or may not be lost, the cutoff degrades network performance and impairs network stability.
Accordingly, there is a need for a self organizing interplanetary communications network for communicating between earth and exploration and data collecting probes, both manned and unmanned and, more particularly, for transporting collected mission critical data with minimum delay and data loss.